to-do: investigate the viability of formatter option for assertEquals()

[iuioiua]

We need to decide whether this option is to be kept before achieving stabilization. See documentation.

One clear point against it is that it's a fourth argument for assertEquals(), which goes against the Deno Style Guide of having two arguments and a third option argument, max (see https://docs.deno.com/runtime/manual/references/contributing/style_guide#exported-functions-max-2-args-put-the-rest-into-an-options-object). If we are to keep it, we should merge msg and formatter into a third options argument.

[kt3k]

Looks like introduced in #3563 , but I don't remember well why this was needed.

I think we should remove that option. We didn't get any feedback about this option. I think nobody uses it.